Arpeggiator work like this in bitwig, you have in what order it should play, it can play chords or up down down up etc.
It moves between the keys you are holding, if you hold c major chord then it moves on these keys. The octave is if you want to add keys like moving 1 octave extra up so it moves on the same keys being hold but also 1 octave up if you have 2 octave active. Hope this makes sense.
Cheers Ronny